Observations of the mechanism of combustion in tobacco compositions such as cigarettes, indicate that the smoke components responsible for biological activity of smoke are formed in the pyrolysis zone of the cigarette cone. The literature suggests that much of this biological activity, observed in connection with the testing of cigarette smoke condensate on standard experimental animals according to conventional protocol, resides in the neutral smoke fraction and more specifically within the subfraction which contains the polycyclic aromatic hydrocarbons (PCAH).
There is a body of opinion that it would be desirable to decrease the levels of PCAH compounds in cigarette smoke and this has led to a substantial amount of research aimed at reducing the proportion of such compounds in cigarette smoke.
It has been postulated that there are several pathways by which the tobacco components are converted into polycyclic aromatic hydrocarbons. One major route is the thermal degradation of various organic materials such as, e.g., cellulose into unsaturated free radical species consisting of two, four or five carbon atoms and, in case of the longer fragments, of conjugated double bonds. The free radical species subsequently participate in the pyrogenesis of aromatic ring structures, the two and four carbon fragments giving rise to unsubstituted PCAH and the five carbon branched structure giving rise to methyl substituted PCAH. Another major route is the formation of PCAH from pre-extant skeletal structures already present in tobacco such as steroids. In the latter case only minor thermally induced modifications are necessary to produce PCAH molecules. Many other routes, such as ring closures of sidechains are possible.
Since the possible pathways of PCAH formation are widely different, it is highly unlikely that any one catalytic agent or other additive would interfere with all of the different formation processes. For instance, in U.S. patent application, Ser. No. 344,589, filed Mar. 26, 1973, now abandoned, by H. G. Bryant, T. Blair Williams and V. Norman, there is disclosed a smoking composition comprising tobacco in association with finely divided metallic palladium or palladium salt. This material is disclosed to result in a tobacco composition wherein the polycyclic aromatic hydrocarbon (PCAH) content arising from the pyrolytic reactions within this composition is substantially reduced when compared to a control cigarette. It has now been found, however, that palladium catalyst alone, while apparently very efficient in eliminating the production of PCAH by some of the pyrosynthetic routes, has its limitations and does not affect all of the pathways.
The addition of nitrates and nitrites to tobacco has been previously described in various patents and publications. Thus, French Pat. No. 1,180,320 teaches the addition of unspecified amounts of nitrites to tobacco and cigarette paper to reduce the PCAH yield and U.S. Pat. No. 3,121,433 describes the addition of potassium nitrate to reconstituted tobacco sheet to improve its burning characteristics. U.S. Pat. No. 3,380,458 teaches the addition of 5.5 to 10% of potassium and sodium nitrates to tobacco (NaNO.sub.3 : 0.91-1.65% nitrate nitrogen, KNO.sub.3 : 0.76-1.39% nitrate nitrogen) and it discloses a reduction in cigarette "tar" yield which is caused by the concomitant increased burn rate of the cigarette.
Bentley and Burgan (Analyst 85, 727-730, 1960) describe the addition of various nitrates to tobacco in an attempt to reduce the yield of 3,4-benzopyrene. They achieved a reduction only with copper and potassium nitrates and increases with lead, silver and zinc nitrates.
Wynder and Hoffman (Acta Pathol. Microbiol. Scand. 52, 119-132, 1961, and Deutch. Med. Wochenschr. 88, 623-628, 1963) using cigarettes treated with 5% copper nitrate (0.50% nitrate nitrogen) confirmed Bentley and Burgan's finding that copper nitrate reduced the 3,4-benzopyrene yield of cigarettes. Hoffman and Wynder also demonstrated (Cancer Res. 27, 172-174, 1967) that the addition of 8.3% of sodium nitrate (1.37% nitrate nitrogen) resulted in a significant reduction of cigarette 3,4-benzopyrene yield as well as in a reduction of the biological activity of the smoke condensate. Pyriki et. al. (Ber. Inst. Tabakforsch. Dresden, 12, 37-55, 1965), on the other hand, have shown that the addition of 4% of potassium nitrate (0.55% nitrate nitrogen) increased the level of 3,4-benzopyrene in cigarette smoke by 40%.
The addition of platinum group metals to tobacco compositions to lower the concentration of benzopyrene in tobacco smoke is disclosed in British Pat. No. 841,074, issued July 13, 1960, to Johnson Matthey and Co. Ltd. The examples of the British reference show only the addition of platinum to tobacco and makes no reference to the addition of inorganic nitrate salts to the tobacco in combination with the platinum.
While most of the past investigators have expressed their research results in terms of the effect of the additive on cigarette 3,4-benzopyrene yield, it is now becoming widely recognized that this compound probably plays at most only a minor role in the biological activity of tobacco smoke condensate. It is also now recognized that the yield of 3,4-benzopyrene, which is a very minor constituent of the PCAH fraction, is not necessarily a reliable indicator of the additives' effect on the bulk of the PCAH.
It has been postulated that the effect of nitrates on the composition of cigarette smoke stems from two properties of nitrates: (a) their capacity to function as oxidants, and (b) their capacity to form the unpaired electron species, nitric oxide, in the pyrolysis zone of the cigarette that acts as a free radical scavenger. Provided a sufficiently high level is added, all nitrates tend to lower the PCAH yield of cigarettes to some degree, but depending on the particular cation, not necessarily the concentration of PCAH in the smoke condensate, as indicated in the Pyriki et. al. article discussed above.